High temperature mechanical behavior of silicon nitride ceramics

J. J. Meléndez-Martínez1, M. Jiménez-Melendo1, A. Domínguez-Rodríguez1, and G. Wötting2

1 Departamento de Física de la Materia Condensada. Universidad de Sevilla

Aptdo. 1065. 41080 Sevilla (Spain) 2 CFI, Ceramics for Industry GmbH & Co. KG

D-96472 Rödental (Germany)

Keywords: silicon nitride, creep, stress exponent, activation energy.

Abstract. The high-temperature compressive creep behavior of silicon nitride ceramics with different densification aids has been investigated. The microstructure of the as-received materials consists on randomly oriented rod-shaped β-silicon nitride grains without α phase. The grains have average dimensions of about 1.5 µm of length and 0.5 µm of width. Creep experiments were conducted in argon atmosphere at temperatures ranging from 1450ºC to 1700ºC and under stresses from 20 to 125 MPa. The stress exponent n and the activation energy Q were measured from stress and temperature changes. Values of n < 1 and Q depending on up- (∆T > 0) or down- (∆T < 0) temperature changes were found. Although no macroscopic failure was observed after relatively large strains (∼30%), cavities developed along grain boundaries. The creep parameters may be consistent with a shear-thickening behavior.

1. Introduction Silicon nitride has become one of the most promising ceramics for structural applications at high

temperatures due to their excellent strength, low density and low coefficient of thermal expansion. Its applications extends from vessels for chemical reactions to heat exchanger and gas turbine components. In this context, it is valuable to measure the creep behavior of these materials at elevated temperatures. Different studies devoted to this subject [1-6] exhibit a wide scatter among them, indicating the strong influence of the second phases present in the materials. In addition, it has been recently shown that fine-grained silicon nitride-based ceramics exhibit a superplastic behavior, with ductilities in excess of several hundred per cent [7,8]. The creep properties have been interpreted on the basis of different deformation mechanisms: solution-reprecipitation, diffusional creep, shear thickening, etc. In the present work, we present a preliminary investigation of the high temperature compressive creep of silicon nitride with small amounts of additives.

2. Experimental procedure Two different materials have been investigated: (i) silicon nitride with Y2O3, Al2O3 and MgO

additives sintered at low pressure (designated N7202); and (ii) silicon nitride sintered at high pressures with Y2O3 and Al2O3 aids (designated N3208). The materials, with a density of 3.2x103 kg/m3, were supplied by Bayer AG (Krefeld, Germany). Paralepipedic samples of 6 x 3 x 3 mm in size were cut from the as-received blocks. Compression creep tests at constant load were carried out in argon at temperatures T between 1450ºC and 1650ºC, under nominal stresses σ between 20 and 125 MPa. One test was performed under nitrogen atmosphere, and no differences with argon environment were found. Stress and temperature changes were performed during deformation in order to measure the stress exponent n and the activation energy Q in conditions of identical microstructures. The microstructural characterization of the as-received and deformed materials was carried out using scanning (SEM) and conventional transmission (TEM) electron microscopy (Microscopy Service, University of Sevilla, Spain). For SEM observations, cross-sections of as-

sintered and deformed samples were cut, ground and polished, and then plasma etched with CF4. Standard ceramographic techniques were used to obtain TEM samples.

3. Results and discussion Fig. 1 shows a SEM micrograph of a N7202 as-received sample. The microstructure consists of

randomly oriented rod-shaped β-silicon nitride grains without α-phase, The average grain dimensions were 1.5 µm in length and 0.5 µm in width. No glass pockets at triple points could be observed at TEM scale. Identical characteristics were found in the N3208 material, except for the slightly smaller grain dimensions (1.3 and 0.3 µm, respectively).

Figs. 2 and 3 show typical creep curves plotted as log ε& vs. ε, where ε& and ε are the strain rate and strain, respectively, with various changes in creep conditions. Macroscopic strains of about 30% were attained without evidence of macroscopic failure. It should be noted that strain rates as high as 10-4 s-1 were reached at the highest stresses and/or temperatures (Figs. 2 and 3), characteristics of a superplastic behavior. The mechanical data have been analyzed using the standard high temperature power law for steady state deformation:

)RT/Q(expA n −σ=ε& (1)

where A is a constant and R the gas constant. The creep parameters n and Q were determined by stress and temperature (∆T = ± 50ºC) changes during deformation, in order to avoid microstructural evolutions.

Fig. 2 shows several determinations of the stress exponent in N3208 samples. The values of n ranges from 0.6 to 1.1, with an average value of 0.8. Similar results were obtained at different experimental conditions, as well as in the N7202 material. To the authors’ knowledge, a stress

Fig. 1. Microstructure of as-sintered silicon nitride (N7202) showing randomly oriented rod-shaped β-silicon nitride grains.

0 5 10 15 20 25STRAIN (%)

STR

AIN

RA

TE (s

-1)

10-3

10-4

10-5

10-6

25 36 47 62 74 89 105 123σ (MPa) =

0.8 0.6 0.6 0.6 0.8 0.9 1.1n =

T = 1650ºCN3208

Fig. 2. Creep curve plotted as log ε& against ε showing several stress changes to determine the stress exponent n.

0 5 10 15 20 25 30 35STRAIN (%)

STR

AIN

RA

TE (s

-1)

10-3

10-4

10-5

10-6

1550 1600Τ (ºC) =

750 430 610 430

Q (kJ/mol) =

σ = 75 MPaN3208

16001550 1550

Fig. 3. Creep curve showing several determinations of the activation energy Q. Positive temperature changes lead systematically to higher Q values.

exponent lower than unity has been only reported by Chen and Hwang [6] in a superplastic SiAlON. The usual n values found in silicon nitride based ceramics range from 1 to 2 [1-8].

The activation energy for creep also exhibits a rather unusual behavior (Fig. 3). Q is systematically higher for positive temperature changes (∆T = 50ºC; average value Q = 650 kJ/mol) than for negative ones (∆T = -50ºC; average value Q = 430 kJ/mol). Grain growth during testing can be ruled out as responsible for this behavior because SEM observations after deformation indicate that the grain dimensions are almost identical to the initial ones. In addition, grain growth would lead to higher Q values in negative temperature changes, which is opposite to the experimental evidence. The activation energies reported in the literature exhibit a wide scatter, with values ranging from 400 to 700 kJ/mol [1,3,7,8]. Kondo et al. [8] have recently reported a Q value of 630 kJ/mol, close to that found in positive temperature changes, in a β-silicon nitride with microstructural characteristics similar to the present materials. However, the stress exponent was about 1.5, characteristic of a superplastic behavior.

Fig. 4 shows the microstructure of a N7202 sample under the SEM after creep up to a final strain ε = 0.3 (T = 1450ºC, σ = 125 MPa). Measurements of grain size indicate that no grain growth occurred during deformation. However, creep damage can be observed, consisting of cavities developed along grain boundaries which occasionally coalesce into small microcracks.

Although it is difficult to asses the role of the creep cavities in the mechanical behavior of the silicon nitride ceramics, it is unlikely that they are responsible for the anomalous low values of the stress exponent and the different up- and down-values of the activation energy. Since these parameters were measured from stress and temperature jumps after relatively short strain intervals (∆ε = 3% – 5%, see Figs. 2 and 3), the cavitation should be much smaller in the initial stages of the deformation than that found at the completion of the test (Fig. 4). A detailed investigation of the evolution of the creep damage with strain is now in progress.

As noted above, Chen and Hwang reported values of n lower than 1 in SiAlONs of very fine microstructures deformed in compression at temperatures between 1500 and 1600ºC [6]. These authors found a transition from n = 1 to n ≅ 0.5 at an almost constant stress of 20 MPa, regardless of the temperature. This behavior was interpreted in terms of a transition from Newtonian behavior

Fig. 4. Microstructure of deformed silicon nitride (N7202) at a final

strain ε = 30% (T = 1450ºC, σ = 125 MPa).

to shear-thickening behavior. Interestingly, such a transition was observed in compression (the method used in the present work), but not in tension [6]. Shear-thickening is a phenomenon commonly observed in the deformation of colloids and polymeric liquids when the viscosity increases with increasing the strain rate. The stresses used in this investigation are higher than 20 MPa, and then may correspond to the shear-thickening regime. In addition, Chen and Hwang found a hysteresis in the stress - strain rate behavior in the shear-thickening regime, which was absent in the Newtonian regime. Such a hysteresis is similar to that observed in Fig. 3 for positive and negative temperature changes. However, the amount of glassy phases in the present materials are much lower than those used by Chen and Hwang. Further studies are required to provide a better understanding of this mechanism and its influence on the creep parameters.

Summary Compressive creep tests have been performed at high temperatures (1450ºC – 1750ºC) in argon

on fine-grained β-silicon nitrides with rod-shaped grains and average grain axial dimensions lower than 1.5 µm. Stress exponents n lower than 1 were found at all experimental conditions by stress changes. The activation energy Q was also determined by temperature changes; positive changes in T systematically gave Q values larger than did negative changes. Cavitation along grain boundaries was observed, but its role on the creep deformation appears minor. Shear-thickening seems to be compatible with the experimental results.

Acknowledgements. This work has been supported by the Brite-Euram project nº BE97-4544.

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